Hydraulics are quite simple in how they work... oil goes in - oil goes out through different ports. When you have problems it's 9x out of 10 it's grit in the system that's either nicked an o ring, scared a shaft creating a leak causing a loss of pressure or grit blocking a port exit. If you gotta do any wrenching on hyds, beforehand clean whatever area you're going to take apart anally with electrical cleaner & pressurized air... the slightest bit of grit can do a world of hurt to hyds!
These china models, as sloppy as their tolerances consistently are & they don't factory install any sort filter either
... a fuel filter for a rc airplane or heli is better than nothing at all. Place it within the returning oil line to the tank. As quick as "they" slap these models together without much care & kick 'em out the door... makes me wonder how thoroughly they clean machining filings from their components as they make them... atleast a filter will catch that crap too so it isn't continually flowing through the system.